fix: Push indices for optional values

When an argument takes an optional value (e.g. `.num_args(0..=1)`), make
sure an index is pushed:
* when a value is specified, for each value
* when no value is specified, for the option, similar to a flag.

Note: this means that a MatchedArg's `num_vals` no longer necessarily
matches the number of indices.

Fixes #2419

clap_builder/src/parser/matches/arg_matches.rs

@@ -834,7 +834,7 @@ impl ArgMatches {
         let arg = some!(self.get_arg(id));
         let i = Indices {
             iter: arg.indices(),
-            len: arg.num_vals(),
+            len: arg.num_indices(),
         };
         Some(i)
     }

clap_builder/src/parser/matches/matched_arg.rs

@@ -67,6 +67,10 @@ impl MatchedArg {
         self.indices.iter().cloned()
     }
 
+    pub(crate) fn num_indices(&self) -> usize {
+        self.indices.len()
+    }
+
     pub(crate) fn get_index(&self, index: usize) -> Option<usize> {
         self.indices.get(index).cloned()
     }

clap_builder/src/parser/parser.rs

@@ -1024,6 +1024,12 @@ impl<'cmd> Parser<'cmd> {
     ) -> ClapResult<()> {
         debug!("Parser::push_arg_values: {raw_vals:?}");
 
+        // If there were no values, treat the arg like a flag
+        if raw_vals.is_empty() {
+            matcher.add_index_to(arg.get_id(), self.cur_idx.get());
+            return Ok(());
+        }
+
         for raw_val in raw_vals {
             // update the current index because each value is a distinct index to clap
             self.cur_idx.set(self.cur_idx.get() + 1);

tests/builder/indices.rs

@@ -179,3 +179,20 @@ fn indices_mult_opt_mult_flag() {
     assert_eq!(m.indices_of("option").unwrap().collect::<Vec<_>>(), [2, 5]);
     assert_eq!(m.indices_of("flag").unwrap().collect::<Vec<_>>(), [6]);
 }
+
+#[test]
+fn indices_mult_optional_value() {
+    let m = Command::new("myapp")
+        .args_override_self(true)
+        .arg(
+            Arg::new("option")
+                .short('o')
+                .num_args(0..=1)
+                .action(ArgAction::Append),
+        )
+        .arg(Arg::new("flag").short('f').action(ArgAction::SetTrue))
+        .try_get_matches_from(vec!["myapp", "-o", "val1", "-f", "-o", "-f"])
+        .unwrap();
+
+    assert_eq!(m.indices_of("option").unwrap().collect::<Vec<_>>(), [2, 4]);
+}